We propose the use of MRI to visualize the thermal interactions of Nd:YAG laser radiation with tissue. Both the temperature dependence of MR relaxation mechanisms and the high sensitivity of MRI to changes in the mobility and distribution of tissue water makes this three-dimensional diagnostic modality particularly suitable for the control of thermal energy deposition in tissues and for the instantaneous spatial detection of laser-induced lesions. The synchronization of pulsed MRI sequences with laser pulses provides a highly efficient way to localize and quantify laser-induced reversible and irreversible changes and to measure dynamic thermal processes. We propose the integration of MR imaging with medical laser techniques. We believe that the combination of lasers and MRI will provide a precise control of thermal interactions in three-dimensional space for safer and only minimally invasive surgical excision of various lesions and for the measurement of tissue reaction to thermal stress. The ability of MRI to perform the exact thermal dosimetry required in hyperthermia is limited, and this proposal is not specifically designed to develop an NMR-controlled laser technique to treat malignancies with hyperthermia. There are potential spin-offs, however, from the application of NMR-controlled laser procedures which may well be of benefit to clinicians using hyperthermia to treat tumors in patients. A relatively small increase in tissue temperature procedures irreversible changes in macromolecular-water interactions which profoundly alter the temperature dependence of relaxation parameters. By determining the presence or absence of hysteresis of the NMR signal intensity vs. temperature curve, reversible thermal changes and irreversible phase transitions can be separated. This distinction of laser-induced reversible and non-reversible changes in tissues can also be achieved without temperature measurement by comparing the transient effect of repeated laser perturbations on various relaxation parameters. In this case, an altered response to identical perturbations will signal the presence of irreversible phase transitions. This approach has relevance not only for relatively high energy laser surgery but also for the characterization of smaller thermal energy-induced changes of normal tissues and tumors.